Formaldehyde-based resins, e.g. Phenol-Formaldehyde (PF), Melamine-Formaldehyde (MF), Urea-Formaldehyde (UF) resins are widely used as nonwovens binder for various industrial applications such as fiberglass insulation industry, paper impregnation, filtration media, and roofing materials. These formaldehyde-based resins are inexpensive, have low viscosity, and are able to cure to form a rigid polymer, thereby providing the finished product with excellent physical properties.
Fiberglass insulation products consist of glass fibers bonded together with covalently crosslinked binder resins. Processes for making fiberglass insulation generally include drawing molten streams of glass to spinning wheels where they are spun into thin fibers by centrifugal force. The fibers are then blown into a forming chamber, sprayed with an aqueous binder and deposited as a mat onto a traveling conveyor. Thereafter, the coated mat is transferred to a curing oven where heated air is blown through the mat to cure the binder and rigidly bond the glass fibers together.
PF resins, typically extended with urea, are widely used throughout the fiberglass insulation industry.
PF resins are also used as a binder for nonwoven filtration media. These filtration products are typically made by a wet-laid technique wherein fibers, e.g. glass or cellulose fibers, are dispersed in aqueous binder slurry. The fibers are then deposited from the binder slurry onto a conventional screen or wire as in a Fourdrinier machine to form a mat, which includes a binder resin, e.g., a phenolic resin.
MF resins are used in manufacturing overlay paper laminates. In general, a porous substrate, such as paper or a fabric web, is impregnated with an MF resin and dried. The dried resin impregnated substrate, along with other layers, are pressed usually with heat to form a laminate.
Glass fiber mats for roofing industry are made by applying a UF-based binder to a wet glass fiber mat, followed by drying and curing the binder at elevated temperatures.
A serious disadvantage of PF, MF and UF resins is high concentration of free formaldehyde, which is undesirable for ecological reasons. During the curing reaction, formaldehyde is volatilized from the binder into the surrounding environment. Although addition of urea to PF resins results in decreasing formaldehyde emissions, at the same time, ammonia emissions and “blue smoke” increase dramatically. Therefore, there is a continuing need for alternative nonwovens binder that would not emit formaldehyde upon curing.
A number of formaldehyde-free compositions have been developed for use as a binder for making nonwoven products.
U.S. Pat. No. 4,076,917 discloses the use of beta-hydroxyalkylamides to cure polycarboxy polymers such as polyacrylic acid. Such a system, however, is too viscous for use as a fibrous glass binder.
U.S. Pat. No. 5,143,582 discloses heat-resistant nonwovens containing ammonia-neutralized polycarboxylic acids, either monomeric or polymeric, and beta-hydroxyalkylamides. However, the binder compositions are believed to liberate ammonia upon cure. Ammonia emissions are becoming increasingly tightly regulated.
U.S. Pat. Nos. 6,221,973 and 6,331,350 describe a formaldehyde-free fiberglass binder including a polyacid, such as polyacrylic acid, and a polyol, with a molecular weight less than about 1000, such as, for example, glycerol, triethanolamine, sorbitol, or ethylene glycol. A phosphorous catalyst is used to accelerate the cure of the composition.
U.S. Pat. No. 5,932,689 describes a curable aqueous composition for fiberglass insulation, which contains (a) a polyacid comprising at least two carboxylic acid groups, anhydride groups, or salts thereof; (b) an active hydrogen-containing compound, such as polyol or polyamine, and (c) a cyanamide, a dicyanamide or a cyanoguanidine. Suitable accelerators include phosphorous or fluoroborate salts.
WO 03/104284 describes an epoxide-type formaldehyde-free insulation binder containing a substantially infinitely water-dilutable or water-dispersable mixture of an epoxide and a multi-functional cross-linker, such as polyamidoamine polymer.
Despite these disclosures, there is a growing need for new formaldehyde-free aqueous compositions suitable for use as a binder for fiberglass insulation, roofing and filtration materials, as well as for paper impregnation.
Some of the drawbacks and limitations of the above-described systems include high cost, high viscosity, low pH causing corrosion of metal parts along the production lines, and high cure temperatures. Thus, new formaldehyde-free binders having lower cost and curing energy requirements similar to those of phenol-formaldehyde resins are desired.
Polyvinyl Alcohol (PVOH) is a water-soluble polymer known to have various uses in view of its excellent properties. PVOH is a polymer with high tensile strength, excellent flexibility, good water resistance and outstanding binding capacity (Finch, C. A., Ed., “Polyvinyl Alcohol: Properties and Applications”, John Wiley & Sons, 1973, pp. 227-230). In view of these advantageous properties, PVOH has been used in the paper processing industry for surface and internal sizing of paper and to impart water resistance to paper. PVOH-based aqueous compositions are also used as coating solutions. However, such systems are generally too viscous for use as a fibrous glass binder.
U.S. Pat. Application No. 20030008586 discloses the use of PVOH as a formaldehyde-free binder solution for low binder nonwoven fiber mat useful for making wood product laminates. The binder produces high bonding strength with wood and is characterized by good storage stability. The binder is used at 5% concentration.
U.S. Pat. No. 6,444,750 describes a curable aqueous PVOH-based coating solution for polymeric substrates. Organic acids such as lactic acid, maleic acid, and citric acid are used as cross-linking promoters. The pH of the solutions is 3.5 or less, which provides substantially 100% cure of PVOH upon drying. The coating solutions exhibit improved viscosity stability. However, formaldehyde-containing crosslinkers are used as the crosslinking agent.
U.S. Pat. No. 6,379,499 describes an aqueous composition for paper treatment, which contains PVOH, a multifunctional aldehyde and a catalyst. Glutaraldehyde and glyoxal are used as cross-linking agent at a concentration of about 50% to about 800% of the weight of PVOH. The aqueous compositions for paper treatment contained about 1% PVOH. After curing the compositions at a temperature between 100° C. and 200° C. for 0.5-5 minutes, the paper demonstrated improved tensile strength and folding endurance.
U.S. Pat. No. 5,354,803 describes a nonwoven binder containing a graft-copolymer of low or ultra-low molecular weight PVOH (12-35%) and a vinyl and/or acrylic monomer (65-88%). The binder is used as a latex (emulsion), which is applied to nonwoven polyester roofing mat. After curing the compositions at a temperature 149-154° C. for 3-5 minutes, the nonwoven products exhibit high temperature resistance, tensile strength and elongation resistance.
U.S. Pat. No. 6,884,849 (hereinafter “the '849 patent”) describes a poly alcohol-based binder composition comprising a low molecular weight polycarboxylic acid and a low molecular weight poly-alcohol, such as PVOH having a number average molecular weight of <7,000. The binder solution preferably comprises at least one cure catalyst or accelerator, such as sodium hypophosphite. The binder exhibits a high cure rate and provides a good recovery of the final nonwoven product. However, a practical use of such a composition for insulation production is limited by its rather low concentration (10-30%) and the high acidity causes corrosion of production lines and problems with cure strength of the final binder product.
While these references and other prior art systems disclose various PVOH-based curable compositions, they have certain limitations with respect to developing a nonwoven binder. A number of these systems have the disadvantage of using formaldehyde-based cross-linkers. Other cross-linking agents release formaldehyde during the cure, for example, N-methylol acrylamide. Further, these conventional systems are used as diluted binders containing, as a rule, 1-5% PVOH. This limitation is due to the high viscosity of aqueous PVOH solutions.
Thus, there is a need in developing new PVOH-based nonwoven binders that could be cured by non-formaldehyde cross-linkers. It is desirable that such curable PVOH compositions contain higher amounts of non-volatiles (sometimes referred to as “NV” hereinafter) (about 25-40% by weight of the resin), and at the same time are stable and infinetely water-dilutable.